Contribution of numeric dynamic modelling to the understanding of the seismotectonic regime of the northern Apennines

In this paper we investigate the forces possibly active in the area of the northern Apennines by means of two-dimensional finite element modelling assuming a viscoelastic rheology. The forces included in the models are related to continental convergence between Africa and Eurasia, to negative buoyancy of the subducted Adriatic lithosphere and to positive buoyancy of anomalously hot mantle material underneath Tuscany. Model-predicted stress distribution is compared with seismotectonic data available for the area of the northern Apennines and with earthquakes distribution. Our results indicate that continental convergence cannot be the only mechanism acting in the study area since it causes insignificant subcrustal stresses, in disagreement with intermediate seismicity observed under the Apennines, Asthenospheric upwelling under Tuscany (back-arc region) is shown to play a crucial role in the present day dynamics of the Tyrrhenian-Apennines. Positive buoyancy causes an upwards and eastwards flow that generates extensional bending stresses in Tuscany and at the top of the slab under the Apennines. Horizontal pushing of the slab is proposed as an alternative mechanism to slab pull to generate subcrustal stresses. Activation of slab pull does not result in a significant change in orientation of the principal stress axes at shallow depths with respect to the model in which only asthenospheric diapirism is active. Therefore, the existence of a continuous gravitationally sinking slab cannot be ruled out on the basis of comparison with seismotectonic observations. Both models successfully reproduce extension in Tuscany and compression in the outer margin of the Apenninic belt, but fail to reproduce extension along the inner portion of the chain. Our modelling highlights the importance of better constraining the lateral extent of the asthenospheric diapir in order to find out whether the extension and tectonic uplift in the Apennines is caused by asthenospheric upwelling as in the area of Tuscany. (C) 1999 Elsevier Science B.V. All rights reserved.